By virtue of their satisfactory mechanical, thermal and chemical properties, polyurethane elastomers are in broad use in various applications including industrial rolls and belts, business machine parts and so on.
For the production of such polyurethane elastomers, there is known a process which comprises reacting an isocyanate-terminated prepolymer, which has been prepared by reacting a polyol with a stoichiometric excess of a polyisocyanate, with a curing agent. In that production process, 4,4'- methylenebis-2-chloroaniline (hereinafter referred to as MOCA) is commonly used as the curing agent. The polyurethane elastomer obtainable with such a known curing agent, typically MOCA, has laudable mechanical and thermal properties but the oncogenicity, recently uncovered, of MOCA caused many apprehensions and its use is now restricted from the standpoint of labor safety and hygiene.
These circumstances gave impetus to research endeavors to find new curing agents that may be substituted for MOCA and so far several compounds including bis(.beta.-hydroxyethoxy)benzene (hereinafter referred to as BHEB), p-xylylene glycol (hereinafter referred to as P-XG) and bis(.beta.-hydroxyethyl) terephthalate (hereinafter referred to as BHET) have been proposed as candidates. However, melting only at high temperatures over the melting point (about 98.degree. C.) of MOCA, these compounds tend to crystallize on blending with the prepolymer, thus failing to provide a homogeneous blend and maintaining the temperature of the system at a high temperature to prevent crystallization presents workability problems. Moreover, the elastomers obtainable with these curing agents are too high in hardness to exhibit the necessary elasticity.
To overcome the above workability problem associated with curing agents, it has been proposed to employ a blend of BHEB with 1,4-butanediol (hereinafter referred to as 1,4-BG) (Japanese Tokkyo Kokai Koho No. 30695/1978) but this proposed process entails a sacrifice of the mechanical property of the product polyurethane elastomer so that the above-mentioned problems still remain to be solved.
It might also be contemplated to blend a low molecular weight triol, such as trimethylolpropane (hereinafter referred to as TMP), with BHEB or the like and use the blend as the curing agent but investigations made by the inventors of the present invention revealed that addition of TMP not only results in a reduced tear strength of the product polyurethane elastomer but as apparent from an increased loss tangent (tan .delta.) in the rubber region in the determination of the temperature variance of dynamic viscoelasticity, a reduced dynamic performance, such as a reduced roll running life.